Insulin has been used clinically for diabetes treatment for decades; protein hormone insulin secreted by pancreatic islet β cells is particularly useful in regulating glucose metabolism and controlling blood sugar balance. Antigenicity of first generation of animal insulin is relatively high as amino acid sequence and structure of mammalian insulin molecule is slightly different; porcine insulin is the closest to human (Baker, E N, Blundell, T L et al. The structure of Zn2+ pig insulin crystals at 1.5 A resolution. Philos. Trans. R. Soc. Lond. B Biol. Sci. 1988. 319:369-456). Molecular weight of second generation human insulin is 5,808 Da, administration dosage can be reduced by approximately 30%; human insulin is relatively stable and can be stored at room temperature of 25° C. for about one month, and is reported to require to be administered 30 mins before meal. Insulin analogues are third generation insulin, which mimic human hypoglycemic action, can be administered just before meal and also known as prandial insulin or rapid-acting insulin.
Active pancreatic islet β cells first secrete proinsulin which made up of 84 amino acids, C chain of the proinsulin is cleaved under the action of pro-protein convertase (PC1 and PC2) and carboxypeptidase E and, C-terminus A chain and N-terminus B chain of the proinsulin join together via disulphide bond to become insulin. Insulin is stored in secretory vesicles of islet β cells as zinc ion coordinated insulin hexamer. Under glucose, lactose, ribose, arginine, glucagon stimulation, insulin is released from secretory vesicles into blood and exerts its physiological function. Insulin that maintains normal fasting blood glucose level is called basal insulin; insulin which reduces blood glucose level after meals to maintain normal blood glucose level is called prandial insulin. Prandial insulin suppresses generation of hepatic endogenous glucose to control the degree and time of blood glucose elevation after meal (Mitrakou A, Kelley D, Veneman T, et al. Contribution of abnormal muscle and liver glucose metabolism topostprandial hyperglycemia in NIDDM, Diabetes 1990; 39:1381-90). As a result, blood sugar level is controlled to near the fasting state level for the majority of time; and blood glucose level above 5.5 mmol/L would not last more than 30 mins (Boehm B, Home P, Behrend C, et al. Premixed insulin aspart 30 vs. premixed human insulin 30/70 twice daily: a randomized trial in type 1 and type 2 diabetic patients. Diabet Med 2002; 19:393-399). Pancreatic β islet cells in type 1 diabetes patients undergo autoimmune destruction which leads to reduction of basal and prandial insulin secretion. Pancreatic β islet cells in type 2 diabetes patients have an abnormal low functional efficiency leading to reduce insulin secretion (Bell D S, Importance of postprandial glucose control. South Med J 2001; 94:804-9), and resistant to insulin; normal fasting blood glucose and post prandial hyperglycemia continue for several hours to next meal (Polonsky K S, Given B D, Hirsch L J, et al. Abnormal patterns of insulin secretion in non-insulin dependent diabetes mellitus. N Engl J Med 1988; 318: 1231-9). Currently, treatment of type 2 diabetes includes analog formulation, such as Novolin N and Insulin Aspart. Type 1 diabetes treatment frequently uses insulin formulation with continuous pulse type insulin releasing pump to maintain a normal fasting blood glucose level. If blood sugar level cannot be effectively controlled through lifestyle and oral treatments, many turns to therapy with a basal insulin analogue, Insulin Detemir.
Byetta/Exenatide is a potent insulin secretion promoting drug, which is use in controlling blood sugar level in type 2 diabetes patients (T2DM), and is particularly useful for metformin, sulfonylurea and metformin sulfonylurea type patients that are more difficult to control blood sugar level. Exenatide has a half life of about two mins (much longer compared with mammalian glucagon-like peptide-1 (GLP-1) (Pridal, et al., Eur. J. Drug. Metab. Pharmacokinet., 21: 51-59, 1996; Deacon et al., Diabetes, 47: 764-769, 1998); is more resistant to proteolytic enzyme IV (dipeptide lactamase IV) degradation; has 53% amino acid sequence identity with GLP-1 (Goke, et al., J. Biol. Chem., 268: 19650-19655, 1993); is able to stimulate GLP-1 receptor, has an antidiabetic-effect related to GLP-1 receptor and has a similar role as GLP-1. Exenatide also enhances glucose-dependent insulin secretion and inhibits secretion of irregular high levels of glucose-dependent glucagon, slows down gastric emptying (Schira, et al., J. Clin. Invest t., 97: 92-103, 1996), reduces food intake, promote β cell proliferation and regeneration, reduce fat accumulation and insulin sensitizing effect (in animal model). Exendin-4 of Gila monster has a long blood sugar lowering effect (Greig et al., Diabetologia 42: 45-50, 1999). When exenatide is subcutaneously administered at 0.2 μg/kg or higher, it causes side-effects, such as vomiting, nausea, headache and the like.
Controlling the initial concentration of exenatide in blood and limiting initial release of exenatide are major barriers in the research and development for a controlled-released formulation of exenatide. U.S. Pat. No. 7,164,005 and Patent Application Publication US2005/0271702 and China Patent Application Publication CN101646424A disclose the use of exenatide containing nano-lipid gel capsule prepared by phase separation of poly(lactic-co-glycolic acid) (PLGA) copolymer. The preparation method includes phase separation method, spray drying method or the like. In addition, China Patent Application Publication CN102274182A discloses use of double emulsion solvent evaporation method to prepare exenatide multivescular liposomes. Although subcutaneous administration of exenatide multivesicular liposomes prepared from this method can control blood sugar level up to 7 days in rat, this method relies on the use of organic solvent and complex formulation that may cause skin irritation and the multivesicular liposome disclosed has a particle size of 5-50 μm. J. Senior et al. (J. Senior, M. Radomsky, Sustained-release injectable products, 1st ed., Interpharm/CRC, New York, 2000) points out that bigger the particle size of a lipid vesicle, the slower the subcutaneous release will be. Other literatures (Chan et al., Clinical Medicine; 2007 (28) 170-173) teach that multivesicular liposomes' particle size is too large for intravenous administration, settlement of liposome aggregate during storage which affects the stability of the formulation remains a problem, and existing liposome products are suspension and thus are difficult to store and transport. Additionally, large production of multivesicular liposome is difficult as the preparation thereof requires stringent production conditions, limiting the development of these liposomes. On Jan. 27, 2012, the FDA approved the weekly administration of exenatide extended-release injection formulation (Bydureon/Exenatide), weekly administration thereof is used as an adjuvant treatment for T2DM. The FDA requires pharmaceutical companies (Amylin) to establish a continuous registration system for severe risk of hypoglycemia, medullary thyroid carcinoma (MTC) and acute pancreatitis to analyze the annual rate of cancers caused by this treatment in the United States for at least 15 years, and to develop a set of risk assessment and mitigation strategies (REMS).
Apart from insulin calculating control pump, an insulin pump also requires an insulin reservoir, receiver tube, special needle and adhesive materials; needles and nozzle must also be changed frequently to prevent infection and blockage. Insulin pump is expensive, thus it is not covered by medical insurance and is frequently paid by patients themselves. Concentration of insulin used in insulin pump is 100 unit/ml, same concentration of human insulin cartridge. The use of insulin pump to administer insulin into subcutaneous tissue and the like, its absorption rate and curve is similar to the general subcutaneous insulin syringe. Decomposition of insulin hexamer into monomer in insulin solution is slow. Short-acting insulin dimer analogues decomposes into monomer rapidly, insulin pump requires administration of a 24 h dose distribution of basal insulin and additional dosage of insulin before meal. Adjustment of dosage is also required to prevent blood sugar level being too low. Insulin dosage also requires to be adjusted according to food intake and time. Adjustment of insulin dose often requires examination of the insulin pump from endocrinologist or service points. Jose M B et al. (Jose M B., Mariko M., Kozo T. et al. Absorption of insulin from Pluronic F-127 gels following subcutaneous administration in rats. Int. J. Pharmaceutics. 1999 (184):189-198) uses Pluronic hybrid hydrogels PF127 for the preparation of a sustained release insulin formulation for subcutaneous administration, but the drug concentration in blood and blood sugar concentration can only maintain for 12 hours in rats, and the formulation fails to provide a good sustained release. However, if merely increase the sustained release effects through increasing the concentration of PF127, skin irritation would increase.
In view of the foregoing, for insulin-dependent diabetic patients, there is a need for a development of a long acting, controlled release insulin formulation that is inexpensive, stable, low irritation to skin and easy for mass production for a safe, effective and compliance treatment.